Failure of Sparcraft Wings on my Aerobatic Aircraft

[pitts]

Well Last week I was on a website called the biplane forum. I posted a topic about a very serious real world problem. I own a pitts S1S aircraft. This aircraft is an experimental aircraft. Not certified. Pitts are very popular airplanes in the aerobatic world. There are many different models that take many different engines that range in hp. The S1S can take from 100hp to 260hp. My aircraft has a modified Lycoming IO360 that puts out 260hp. The airframe is supposed to basically withstand 2500hrs of aerobatic flight with some minor repairs through its life. Because of the popularity of this aircraft there are many companies that make many aftermarket parts for them. I happen to own a set of aftermarket wings from a company called SPARCRAFT. This company came out in the 1970's. Back then the ribs where flat routed plywood ribs. These wings had a tendency to come apart. Needless to say the company went out of buisness. Recently the company reopened under a new owner. They changed the plwood ribs with built up truss style compression ribs. Now they say they are better than the factory designed wings. When an aircraft is new and experimental it has to fly for 25hrs to complete is test flying and receive an airworthiness certificate. My aircraft flew aerobatics from 25hrs to 70 hrstt. Now my wings are unairworthy. A pitts is a biplane. The upper wing has a fuel tank mounted in the center between two spars that ron the length of the wing. It has compresion ribs throughout. The wings have a total of 4 ailerons. There are 2 I struts that connect the upper wings to the lower wings. The lower wings also have two spars and compression ribs. The lower wings mount to the sides of the fueselage. The upper wing also has a center cabane support in front of the cockpit attached to the center of the upper wing. There are also 4 brunson or flying wire braces that run like an x pattern (if standing in front of the aircraft looking at the wings)for more cross bracing. The wings are covered in ceconite fabric. The first problem I had was the fabric on the top side of the wing kept tearing next to the fuel tank on both sides. Then the rib lacing (thread that penetrates the top and bottom layer of the fabric wrapped around each rib in 4" intervals) began tearing. The final straw that made them unairworthy is there is a vaccumed formed plywood leading edge. On top of every rib the leading edge cracked through and through. I am a pretty knowledgeable aircraft mechanic so I noticed the damage occurring. But to the regular pilot the odds of a person getting in serious trouble are great. 100's of these wings have been sold to the general public. I haved reported them to the faa. I tried to make this information known on the biplane forum. Most of the pitts builders converse there. The owner of the sparcraft company threw a fit and they erased my post and banned me from that site in under a day and a half. I would like to know if there is an aeronautical structural engineer who could calculate how much g-force those wings can handle if I gave them tall the hard numbers. Airfoil dimensions gross weight prop diameter pitch rpm wing loading wing area. There are also 3-d drawings available to the public on www.steenaerolab.com That is who owns the rights to the original pitts S1S design. Please write back and I will add all the information I can muster up.

 

[FredGarvin]

Pitts are pretty sweet little aircraft. I've seen them many a times. This scenario is a scary reason why I always cringe when I hear "aftermarket" for any aircraft.

This is not an easy scenario to examine. Basically it can come down to two problems:
1) Structural deficiency
- Basic structural loading deficiencies
- Fatigue issues
2) Manufacturing problems (where I would put my money at this point)

Either way, you probably will not dig up the problem with a basic load analysis unless the supplier is extremely negligent (right now I would give them the benefit of doubt).

The other problem is going to be wrapped up in the whole "experimental" tag for the aircraft. I'm not sure where that leaves the aftermarket supplier of replacement parts. Does that mean that they have to have their designs OK'd by the FAA and a DER? I don't know on that part. I'd have to do some digging. I do know from my days in large aircraft component testing, that aftermarket parts had to demonstrate the same confidence levels as the OEM components.

I would think your number one item on the to-do list would be to find out if anyone else having these issues. Perhaps is there a way to get back on that board and rephrase your thread to simply ask the fellow S1S owners with these wings if they had done the same inspection you performed? Don't say anything about inferior products, etc...

 

[pitts]

Well on that website there is no way to reach the forum moderater. I am currently suspended and with no way to reach anyone I do not now how long it will last. Other people probably have not had the same problems yet. Reason being the wing kit has not been out for even 2 years yet. For someone to complete an entire aircraft in a 6 month period they would have to be a proffesional builder like myself. I use all the latest equipment. I have a laser dyn I purchased from P&W as well as a few turning centers. I made jigs for this aircraft as I have done with all the aircraft I have built. Usually I can cut standard build time in half. I buy as much of the components pre made as far as fittings, hardware, linkage, and what not. I also work 12 hrs a day seven days a week. I highly doubt anyone has reached the point of completion and test flying. As well as 45 hrs of just acro flight. I know fatigue can also be ruled out. I spoke to steens engineer as well. He said the problem lies in the rear spar. I have thought about that as well and I also believe that is the problem. The rear spar is not attached to the fueselage. Only the forward spar is. Which means that there has to be some sort of cross bracing in the open boxed areas of the wing. The only part there is the plywood wing walk which only extends out two stations. The other thing that really leads me to believe this to be the problem is when I was on the biplane forum sight a response was written to me well lokk at so and so's wings who flies unlimited acro in competition. His wings have held up. Well I looked at pictures of these wings and they actually had steel cables taught between each station, and they had added all kinds of extra pieces. I just want to provide a legitamite load breakdown to show in scientific engineering nature that these wings will fail so the FAA will act on this before fatalities occur.

 

[berkeman]

A little visual aid...

http://farm1.static.flickr.com/94/238548991_95c877e9ce.jpg?v=0

 

[pitts]

Pictures... construction photo's are not the same as the actual kit. That is what these wings need to be safe...The pro's who have been breaking airplanes doing acro know this from experience. The general public however does not. I know a lot about building aircraft but I do not know about structural design. G-force is hard to calculate without an engineering degree. The FAA always encourages not to vary from blueprints. The normal aircraft builder does not seek the advice of an aeronautical engineer about engineered blueprints signed off by the FAA.

 

[Cyrus]

Very cool thread. Sorry I don't to aerospace structures or I would offer to help. Seems like a pretty scum-bag thing this company did to get your posts taken down. Keep us updated on what the FAA says though!

 

[mugaliens]

I just want to provide a legitamite load breakdown to show in scientific engineering nature that these wings will fail so the FAA will act on this before fatalities occur.

I hear you, pitts. However, even for a small plane like the Pitts, reverse engineering the wings would take hundreds, if not thousands of hours. Blueprints would be a big help, of course, but it would still take taken a great deal of time.

To compound this, determining current structural integrity given the problems you mention would be almost impossible to accomplish computationally.

The more appropriate approach is document the issue with clean, sharp pictures and well-written documentation, before sending a package with a good cover letter to the FAA.

I wouldn't give the website another thought, as most such are marketing and builders support functions for the company, and they'll right you tooth and nail on any negative information you might have.

 

[dr dodge]

Is the EAA of any help?
I would consider starting my own web site.
(if I was mad enough I'd start operation "bad press" too)
pictures of the failures, a place for other people to log complaints
buy the domain name, don't be accusational, but build a case.
purchase ad space in some mags
unfortunately, a "friend in the gov't" had once told me FAA doesn't do much without people getting hurt or dying, which is unfortunate.

dr